help: use command-list.h for common command list

help: use command-list.h for common command list

The previous commit added code generation for all_cmd_desc[] which
includes almost everything we need to generate common command list.
Convert help code to use that array instead and drop common_cmds[] array.

The description of each common command group is removed from
command-list.txt. This keeps this file format simpler. common-cmds.h
will not be generated correctly after this change due to the
command-list.txt format change. But it does not matter and
common-cmds.h will be removed.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>